The present invention relates to drive circuits that provide AC power (or in some cases DC power) to a motor armature or other AC load or to a reversing load, and is more particularly directed to a control circuit that provides AC power at a frequency, wave shape, and power factor that is tailored for an AC or inductive load that may vary during use, such as a single-phase AC induction motor, an example of which is a motor used to drive a compressor or blower in a HVAC application. The invention is more specifically concerned with a control circuit for applying drive power to a motor which may be from fractional horsepower to several horsepower or above in size; in which a torsional load varies depending upon external factors, and in which the motor torque can depend on the condition or quality of the AC line power; and in which the applied line voltage can drop from a nominal 117 volts (RMS) to below 100 volts, in which the power factor or phase angle can be significant; and in which the line frequency may drop well below the nominal 60 Hz (e.g., where the power is locally generated). Such drive power control circuits may have numerous industrial, commercial, and domestic applications.
In refrigeration and HVAC systems, it is often necessary to change the fan speed, blower speed or compressor speed to satisfy changing environmental conditions. For example, because cold, dry air is considerably heavier than warm moist air, during initial operation the blower has to operate at high speed to pump conditioned air, especially to higher floors. Then, when the comfort space or living space has cooled down, the fan speed is reduced to avoid blowing cold air directly on human occupants. Also, where sensible cooling is needed, rather than latent cooling the blower is operated at higher speed to increase air flow. Correspondingly, if dehumidification is required more than sensible cooling, the air flow rate should be reduced, requiring a slower blower speed. Likewise, as the demand for cooling changes, the need for liquid refrigerant through the system will also vary, and so compressor speed may need to be increased at times and reduced at other times.
Also, during many peak demand times, the quality of the AC line power can vary enormously, with changes in line voltage and line frequency. Typically, the motor designer is forced to over-design the motor in order to satisfy load requirements over an expected range of input conditions. The motor armature, which is basically an inductive load, may have to face an unfavorable power factor, which means that the actual applied voltage, i.e., the real component of the complex AC voltage, may become unacceptably low. Consequently, it is desirable to be able to adjust the shape and RMS value of the line voltage so that the motor will operate optimally, even under adverse line conditions.
It is well known that for an AC induction motor, the output torque is proportional to the square of the input voltage. It is also the practice for any given application to use a motor that is rated over a given voltage range of xc2x110%. This means that the system has to be over-designed to meet full load requirements at low voltage. Otherwise, for a given AC induction motor, if the input voltage is 10% low, i.e., V=90% Vnormal, then output torque T drops to T=81% Tnormal. This means that, according to conventional practice, the motor has to be over-designed by at least 19%. Consequently, at normal or high line conditions, over 20% of the electric energy is wasted.
One approach to motor control, which was intended for create control over motor speed, has been an adjustable speed drive (ASD) employing a pulse-controlled inverter. In these ASD""s the incoming AC power is rectified to produce a constant DC level, and that is converted to an AC drive wave using pulse-width modulation (PWM). These ASD""s overcome the shortcomings of operating induction motors directly on line voltage, and satisfy many of the requirements for speed control.
In many cases, what is needed is simply to boost (or to regulate) the effective RMS voltage. This can permit a smaller-rated motor to be used, than would be recommended where unmodified line power is applied directly to the motor.
Also, previous efforts in brownout protection (i.e., to protect the AC induction motor from burn out in low line voltage situations) have taken the approach of cutting off power to the motor to prevent damage. While this saves the motor, it can cause severe problems for the system that the motor is designed to drive. For example, in a commercial refrigeration application, a freezer system can be used for storage of a frozen food product, e.g., frozen meat, ice cream, or another food product. During a so-called brownout, when the operating line voltage drops below a safe threshold (e.g., reduced from 120 volts RMS to below 95 volts RMS) then the compressor motor is simply shut off, and no refrigeration takes place. If the brownout lasts for a period of an hour or more, the meat may begin to spoil, or the ice cream may melt. It would be more desirable to continue to operate the refrigeration system during brownouts, i.e., by modifying the AC power wave so that it is sufficient to run the equipment, even if at a partial speed. However, that has not been possible with existing power control circuits.
It is an object of this invention to provide power to an AC load, such as a motor armature, and to regulate and control the drive waveshape and frequency in a way that avoids the drawbacks of the prior art, as mentioned above.
It is an object of this invention to optimize applied power to avoid waste of electrical energy.
It is another object to provide a motor speed control for efficient operation of a compressor motor, blower motor, or other non-linear or reactive load.
It is a further object to provide a motor speed control that is reliable and efficient, and which can accommodate changes in load and changes in line power quality.
If is a yet further object of this invention to detect or measure dynamically the speed of the rotor, and to adjust the power level to match load requirements accordingly.
It is a still further object of this invention to expand the voltage operating range of existing or new equipment above and below the nominal frequency and voltage of the AC line.
If is a further object of this invention to optimize the voltage-to-frequency ratio of an AC induction machine or motor to enhance system efficiency by dynamically sensing rotor slip, monitoring motor current or back EMF amplitude and/or duration, and adjusting power levels accordingly to match load requirements, for both linear and non-linear loads.
It is still another object of this invention to provide a circuit of low-component count, low-loss, and low-cost design.
According to one aspect of the invention, a power controller for powering an AC single phase induction motor, three-phase induction motor, or other AC load (or in some cases a DC motor) employs input conductors that connect with a source of AC line power, the line power having a waveform and a line frequency, and output conductors that connect to an AC load, such as an induction motor. A variable drive circuit receives the line power from the input conductors and delivers properly conditioned AC power via output conductors to the AC load. The variable drive circuit can be any convenient drive circuit, such as PWM (pulse width modulated) and can be, e.g., of a full bridge or half-bridge design. The principles of this invention can also be embodied in a single-MOSFET switched bridge circuit, in which the armature of the AC induction motor (or other load) is connected in series with the AC inputs of a diode bridge, and which can be realized in single-phase or polyphase modes. The pulse frequency and amplitude can both the controlled, so that the power that is applied to the load at a has a magnitude and frequency that is optimal for the system demands and load conditions. These can be higher than, the same as, or lower than the line voltage and frequency. The variable drive circuit can employ a control circuit that has sensor inputs coupled to the load for monitoring and controlling the waveform and frequency of the power applied to the load. As explained later, the load itself serves the sensor, i.e., detecting the amount of rotor slip in the form of back EMF amplitude and/or width.
The control circuitry of this invention can be of a simple, straightforward design, satisfying requirements of being inherently compact and relatively inexpensive, and at the same time avoiding energy waste.
The above and many other objects, features, and advantages of this invention will become apparent from the ensuing description of a preferred embodiment, which should be read in conjunction with the accompanying Drawing.